In some conventional systems, an RF signal may be converted to an IF, and then from IF to a baseband signal where the IF may be in the megahertz range. Generally, the RF signal may be mixed with a local oscillator signal that may result in two sideband signals that may be the sum of the frequencies of the two signals and the difference of the frequencies of the two signals. One of the two sideband signals may be chosen as an IF signal, and this IF signal may be the same for all received RF signals. A radio that may receive a plurality of channels, such as an AM or FM radio, may tune to a particular station by changing the local oscillator signal frequency such that the IF remains constant. With a constant IF, most of the receive path may be common in the receiver. In some systems the signals, which are digitally processed in the receiver, are baseband signals. As such, the received IF signals must first be down converted to baseband.
Today, much of radio receiver development may be driven mostly by a great demand for mobile wireless communication devices, including handsets. With the ever-decreasing size of mobile handsets, capacities of smaller batteries may be an issue. As most of these handsets may use complementary metal-oxide semiconductor (CMOS) technology for analog to digital conversion, and for much of the processing of voice and data signals, a very important factor to consider is that it may be advantageous for CMOS devices to work at lower frequencies. This may be crucial since CMOS devices have power dissipation directly related to the speed at which the CMOS devices switch. The faster the frequencies, the faster the CMOS device switching speed, and therefore, the greater the amount of power consumed. The receivers may be designed to downconvert the high frequency RF, which may be in gigahertz range, to a lower frequency, preferably to a baseband frequency, as quickly as possible.
The process of downconversion of received signals from IF to baseband may have some drawbacks, such as DC-offset generation, interference noise, I/Q mismatch, excessive flicker noise in the baseband, and local oscillator (LO) leakage. In addition, a digital signal processor (DSP) may be required to perform complex digital processing of the digital signal for filtering and downconverting from the RF frequency. If a measured signal-to-noise ratio (SNR) is less than a desired SNR, the DSP may need to perform, for example, distortion cancellation or other SNR reduction or mitigation function. Additionally, during the process of IF to baseband downconversion, a signal path's intrinsic DC offsets may be amplified. The dynamic range of the circuit may thereby be degraded. In addition, a DC offset may be created if the LO signal leaks to the RF front end and self-mixes. Some systems, for example, Global Systems for Mobile communications (GSM) systems, may use modulation and system synchronization techniques that require DC information, therefore, it may not be feasible to simply remove the DC component. Once a signal is passed from the IF to baseband downconversion stage devices and systems which comprise later stages in the processing of the signal will be unable to distinguish the DC level which was contained in the originally received signal, from the DC offset that was introduced during IF to baseband downconversion.
Further limitations and disadvantages of conventional and traditional approaches will become apparent to one of skill in the art, through comparison of such systems with some aspects of the present invention as set forth in the remainder of the present application with reference to the drawings.